The invention relates generally to laser sources and more particularly to techniques for defining a gain spectrum of available laser emissions and for wavelength tuning within the gain spectrum.
Optical communications and networking systems utilize tunable laser sources to produce laser emissions at selected wavelengths. Advantages of a tunable laser source include an increase in the flexibility of the system and an increase in the information-transfer capability of the system.
A tunable laser source includes a number of components. A conventional laser having a fixed wavelength emission may be used to generate a light beam which is pumped into a laser gain medium that enables broad spectral emission over a useful range of wavelengths. A wavelength-selection component (e.g., a tuning component) is employed to preferentially select a wavelength from the range for encoding and carrying information.
Operations for encoding a laser beam to carry information may take place at the beam-generating stage by the timing of a switching mechanism that activates and deactivates the pumping of laser light into the gain stage. However, the encoding operations typically occur after the wavelength-selection stage that follows the gain stage. Either arrangement operates well for its intended purposes. However, what is needed is a method and device for increasing the compactness of tuned laser sources. What is further needed is such a method and device for enabling a broadening of the gain spectrum in a cost efficient and reproducible manner.
A compact tunable laser source or tuned laser source is obtained by using rare earth doping techniques and holographic imprinting techniques to define a laser gain spectrum and to select a wavelength from within the gain spectrum. Broadening of the gain spectrum may be achieved by establishing varied domains of space charge within the rare earth-doped laser gain medium in which the gain spectrum is defined. Flexibility in the selection of a wavelength can be obtained by holographically imprinting multiple sets of wavelength selection elements within a single segment of the tunable laser source. Compactness can be enhanced by fabricating the doped laser gain medium and the tuning segment as a unitary member, such as by introducing a rare earth dopant and wavelength selection elements into a single substrate of photorefractive crystal.
The operations of the laser gain medium are triggered merely by introducing laser light into the medium to excite quantum states. As is well known in the art, the laser output of a tunable laser source is a function of the available specific energy gaps between higher and lower quantum states, which are based upon the nature of the gain medium. High gain and broad spectrums of useful wavelengths (e.g., wavelengths which are compatible with fiber optic communication systems) are desirable. The use of the rare earth dopant produces the desired wavelength band of emission. Acceptable dopants include one or more of Er, Yb and Nd.
As noted, one aspect of the invention is to provide artificial broadening of the gain spectrum by forming variations in the domains of space charge within the rare earth-doped gain medium. By forming the gain medium within photorefractive crystal, variations in domains of polarization can appear to be randomized merely by using laser writing during the fabrication process. Alternatively, randomized domains of polarizations can be achieved by thermally annealing the photorefractive crystal. As a third possibility, the introduction of impurities into rare earth-doped gain medium may provide the desired domain randomization. Thus, this spectrum broadening aspect of the invention is particularly well suited for use in the embodiment in which the laser gain medium and the wavelength selection elements are formed in a single photorefractive crystal.
Holographic gratings can be imprinted into the photorefractive crystal during fabrication of the tunable or tuned laser source. Multiple laser beams may be directed into the crystal at intersecting angles to form interference patterns. A high electric field is established within the crystal to alter the mobility of carriers, thereby allowing periodic polarization (or space charge) patterns to develop in accordance with the interference patterns of the laser fields. A number of different sets of holographic gratings may be formed within the photorefractive medium using this approach, with each set being specific to a different center wavelength within the gain spectrum of wavelengths. In one embodiment, each set of holographic gratings is tunable within a range about its center wavelength, but the sets may be fixed with respect to selecting their respective center wavelengths.
After the tuning elements have been holographically imprinted, a combination of electrical and thermal conditions determines the diffraction characteristics of a set of holographic gratings and/or determines which set of holographic gratings is activated. Typically, the temperature is stabilized, so that the sets of gratings are tuned by varying the electric field. Each set of gratings functions as a Bragg grating to preferentially pass or reflect light on the basis of wavelengths. In one embodiment, the Bragg gratings are designed to reflect only the desired wavelength of laser light, while the unselected wavelengths are passed. This is the preferred mode of operation for the case in which the gratings function as an output coupler. However, the opposite arrangement is also a possibility, since both preferential reflection gratings and preferential transmission gratings are known in the art.
An advantage of the invention is that a compact laser source is formed using techniques which are reliable and repeatable. The rare earth doping produces a gain spectrum which includes wavelengths used within the optical communications environment. A gain spectrum of wavelengths between 1300 nm and 1600 nm is preferred, with a range of 1530 nm to 1560 nm being most preferred. By incorporating a number of sets of holographic gratings within the same material that includes the region that is rare earth doped to define the laser gain medium, a flexibility in the selection of the frequency of the laser output is significantly increased.